This application relates to oscillators including opto-electronic oscillators.
An opto-electronic oscillator (OEO) may be designed to include at least one closed loop to generate a desired oscillation signal. Some examples of such an OEO are described in, e.g., U.S. Pat. No. 5,723,856 to Yao and Maleki, U.S. Pat. No. 5,777,778 to Yao, U.S. Pat. No. 5,929,430 to Yao and Maleki, and U.S. Pat. No. 6,567,436 to Yao, Maleki, and Ilchenko. In these examples, an OEO includes an electrically controllable optical modulator and at least one active opto-electronic feedback loop that comprises an optical part and an electrical part interconnected by a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The loop produces a desired delay and feeds the electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillation in radio frequency spectrum when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss.
OEOs use optical modulation to produce oscillations in frequency spectral ranges that are outside the optical spectrum, such as in the RF and microwave frequencies. The generated oscillating signals are tunable in frequencies and can have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators. Notably, the OEOs are optical and electronic hybrid devices and allow for both electrical and optical outputs which can be used for versatile applications.
The optical part of the opto-electronic feedback loop can include an optical delay element to produce an optical delay which constitutes a part of or the majority of the total delay produced by the feedback loop. The optical delay element can be implemented in various configurations. Notably, a high-Q optical resonator may be used as the optical delay element and be disposed in the optical part of the opto-electronic feedback loop or in another optical feedback loop coupled to the opto-electronic feedback loop, to provide a sufficiently long energy storage time and an optical filtering mechanism to produce an oscillation of a narrow linewidth and low phase noise. The mode spacing of the optical resonator is equal to one mode spacing, or a multiplicity of the mode spacing, of the opto-electronic feedback loop. In addition, the oscillating frequency of the OEO is equal to one mode spacing or a multiple of the mode spacing of the optical resonator. The above cited U.S. Pat. No. 6,567,436 describes examples of OEOs with at least one optical resonator in the optical part of the opto-electronic feedback loop.
The optical resonator may be implemented in a number of configurations, including, e.g., a Fabry-Perot resonator, a fiber ring resonator, and an optical resonator operating in one or more whispering-gallery modes (WGMs) such as microsphere, microdisk, and microring WGM resonators. In particular, non-spherical WGM resonators may be used. These and other optical resonator configurations can reduce the physical size of the OEOs and allow integration of an OEO with other photonic devices and components in a compact package such as a single semiconductor chip.